Simon Channing Williams played a major part in the British film industry by producing all the films directed by Mike Leigh for the past 20 years. In 1988 he and Leigh formed Thin Man Films, the company that funded and made possible Leigh's projects, including the multi-Oscar nominated Secrets and Lies (1996) and the equally acclaimed Vera Drake (2004). Channing Williams also produced the controversial story of a whistleblower, The Constant Gardener (2005), directed by Fernando Meirelles, which won Rachel Weisz a supporting actress Oscar for her portrayal of an activist whose uncovering of corporate corruption – the testing of drugs on Africans by pharmaceutical companies – leads to her murder.

Born in Maidenhead, Berkshire in 1945, he was the son of Major-General John Channing-Williams, who had won the DSO for gallantry at the Normandy landings. Channing Williams later dropped the hyphen from his surname. After attending Stowe School, where he enjoyed performing in school plays, he acted with repertory companies for several years prior to joining the BBC in the 1970s as a captioning assistant.

He had risen to the post of assistant director when he first met Mike Leigh, who had written the play Grown-Ups (1980), which was transmitted in the BBC2 Playhouse series, with a cast that included Brenda Blethyn (who was 16 years later Oscar-nominated for her role in Secrets and Lies). Leigh was impressed by Channing Williams's flair for organisation, and a lifelong friendship was formed. Leigh had already written such hits as Nuts in May (1976) and the landmark Abigail's Party (1977) for BBC1's remarkable Play for Today slot, but he had made only one feature film, Bleak Moments (1971), a study of loneliness that was all too aptly titled for general audiences despite the moments of acute insight that were later to become Leigh's trademark.

By 1988 Channing Williams had been associate producer of Tony Palmer's television film Wagner (1983), starring Richard Burton, and his credits as assistant director included Hugh Hudson's film Greystoke: The Legend of Tarzan, Lord of the Apes (1984), and an episode of Widows 2, the television mini-series written by Lynda La Plante. The same year he produced Palmer's Puccini for televison, and in 1985 he was associate producer of seven episodes of Minder, but he wanted to move on.

In 1987 Channing Williams and Leigh worked together again, making a short film The Short and Curlies, starring Alison Steadman and David Thewlis, and in 1988 they formed Thin Man Films (so named since both men were far from slim), and began an impressive series of movies written and directed by Leigh, with Channing Williams energetically raising the cash required and securing distribution. "He loved the cut and thrust of negotiations," said Leigh.

The first film the team made was High Hopes (1988), a satirical dissection of Margaret Thatcher's Britain which skilfully blended the bleak lives of working-class misfits with unexpected moments of comedy, and it proved surprisingly successful in the US. It was followed by such films as Life is Sweet (1991), Naked (1993) and Career Girls (1997), all basically written and directed by Leigh, though he is famous for starting with virtually blank paper and allowing his cast members to improvise, a method with which Channing Williams was reportedly perfectly comfortable.

Secrets and Lies proved a mainstream hit and won the Palme D'Or, and the equally masterly Vera Drake, the moving story of a well-meaning backstreet abortionist, superbly played by Imelda Staunton, won the Golden Lion at Venice. Topsy-Turvy (1999), a lush and affectionate depiction of the operetta writers Gilbert and Sullivan and their relationship while writing The Mikado, was an atypical project that won Oscars for its costume design and make-up. The last Leigh film to be produced by Channing Williams, Happy-Go-Lucky (2007), starring Sally Hawkins, was noted for having an exceptionally upbeat tone for a Leigh project.

Channing Williams individually produced a few other titles, such as When the Whales Came (1989), made on the Scilly Isles, and Jack and Sarah (1995). In 2000 he formed another production company with Gail Egan, Potboiler Productions, their varied output including a successful adaptation of Nicholas Nickleby (2002) along with a misguided biography of the composer Cole Porter, De-Lovely (2004), a clumsily constructed piece that was able to depict Porter's gay affairs but was otherwise even more inaccurate than the much derided Hollywood biopic, Night and Day (1946).

While filming The Constant Gardener, Channing Williams was instrumental in setting up the Constant Gardener Trust, which is building a secondary school in Loiyangalani, the town near Nairobi that was used for location work, as well as building showers and toilets in Nairobi slums and installing 10,000-litre water towers. Most recently, Channing Williams and Egan produced Blindness (2008), starring Julianne Moore, and they had recently bought the rights to John Le Carré's novel, The Mission Song.

Although he had been seriously ill for five years, Channing Williams, who is survived by his third wife, the costume designer Annie Long, also ran two public houses in Berkshire. "He battled cancer very bravely," said Leigh. "His great phrase was, 'Let's just get on with things' and right till the last he was still working. He was a very extraordinary, big man and he will be missed universally."

Tom Vallance

Simon Channing Williams, film producer: born Maidenhead, Berkshire 10 June 1945; three times married (five children); died Penzance, Cornwall 11 April 2009.